Automatic injection devices offer an alternative to manually-operated syringes for delivering therapeutic agents into patients' bodies and allow patients to self-administer injections. Automatic injection devices have been used to deliver medications under emergency conditions, for example, to administer epinephrine to counteract the effects of a severe allergic reaction. Automatic injection devices have also been described for use in administering anti-arrhythmic medications and selective thrombolytic agents during a heart attack (See, e.g., U.S. Pat. Nos. 3,910,260; 4,004,577; 4,689,042; 4,755,169; and 4,795,433). Various types of automatic injection devices are also described in, for example, U.S. Pat. Nos. 3,941,130; 4,261,358; 5,085,642; 5,092,843; 5,102,393; 5,267,963; 6,149,626; 6,270,479; and 6,371,939; and International Patent Publication No. WO/2008/005315.
Conventionally, an automatic injection device includes a housing that houses a syringe and, when operated, causes the syringe to move forwardly within the housing and a needle to project from the housing so that a therapeutic agent contained in the syringe is ejected into a patient's body. An automatic injection device typically includes a plunger with a distal end that is seated on a firing body before firing. In order to fire the device, a patient depresses a firing button which disengages the distal end of the plunger from the firing body and allows the plunger to move the syringe forwardly. An automatic injection device may include a lockout shroud that is deployed during or after an injection to provide a protecting covering over the needle and to thereby prevent accidental needle stick injuries to the user.
Certain conventional automatic injection devices experience problematic shroud deployment including, but not limited to, complete failure in shroud deployment, incomplete shroud deployment, and complete or incomplete shroud deployment after an unacceptably long delay, and the like. Each of these problematic shroud deployment patterns may be referred to as shroud deployment failure or failure in shroud deployment. Shroud deployment failure is undesirable in automatic injection device as they can introduce the risk of accidental needle stick injury caused by an exposed needle.